The present invention is related to optical networks, and in particular, to low-cost, general-use optical networks suitable for transmission of traffic using Internet Protocol (IP).
Prior optical network technology was designed primarily for voice communication. Such voice communication (telephone circuits, etc.) employed a guaranteed service mode in which a complete communication path between the users at both ends was guaranteed for the duration of the call. In this guaranteed service mode, specific users were guaranteed a specific bandwidth for the duration of the call, regardless of actually transmitted signals. Such guaranteed service required the construction of highly reliable and expensive optical networks with redundant paths to provide immediate recovery from fault conditions. Increased volume of communications over telephone circuits has advanced speed and capacity in optical network.
In recent years, data communication using Internet Protocol (IP) has experienced explosive growth and has rapidly replaced guaranteed service-type telephone circuits as the primary mode of communication. In Internet Protocol, when a signal ‘packet’ or a small chunk of data arrives a router, the router routes the packet to an open transmission path. This technique, in which a fixed communications path need not be established between end users is called a ‘connectionless’ network protocol. Connectionless systems have reduced costs because multiple users share the same signal bandwidth. This system also features rapid recovery from faults. When a fault occurs in a given path, after short delay to adjusted signal flow between routers, the affect signals are routed through a different path.
Because data communications systems using Internet Protocol are capable of handling multimedia signals, there has been a growing demand in data capacity not only for text, but also for audio, graphic images, and video for personal computer communication. Because the growing demand, greater data transmission capacity as well as flexible expandability will be required. Also, the areas where data is transmitted have been expanding. Along with the need to connect more users over longer distance, there is also a growing demand for high capacity optical transmission of 100 Mbit/s-1 Gbit/s over distances ranging from a few tens of kilometers to a few hundred kilometers. In the past, optical transmission methods have primarily concentrated on guaranteed service-type communications links as generally used for telephone circuits to provide large capacity, high reliability, and high quality service. Prior technology has difficulties in providing cost reduction and flexibly responding to client demands.
From the start, large scale optical networks have included ring, bus, and star configurations. In general, optical network systems had to be capable of high-speed, high-capacity, high-reliability, and high-quality data transmission. For these reasons, to meet the speed and capacity requirements, optical network system designers tended to opt for techniques such as load distribution and function distribution. Also, to meet the reliability requirements, they used redundancy such as duplicate circuits, hot standby circuits, etc. while to meet the quality standards, they used QoS (quality of service) processes and TCP (transmission control protocol).
In the past, because their designers focused primarily on obtaining extremely high-speed, high-capacity, high-reliability and high-quality signal transmission, optical telephone networks tended to be too expensive for widespread use. The use of Internet (Internet Protocol) traffic has required less speed, capacity, reliability, and quality in internet-type communication than the conventional voice communication circuits. A practical optical network configuration is desired for Internet Protocol communication in mid-sized networks such as for use in covering metropolitan areas up to 200 km. A low-cost optical network configuration is also desired with sufficient reliability for data communication-using Internet Protocol without abnormal congestion under fault conditions. Also, in the past, because it was voice communication that determined the standards for telephone circuit networks, the required number of subscriber lines for such networks could be accurately predicted from the number of residences and offices in a given service area. However, because data networks handle everything from simple text to high quality video, networks now must have sufficient flexibility for expandability so as to accommodate a broad range of traffic volume at low cost. It is desired that optical networks have low initial costs, but are easily upgradeable for an operator to have a good cash flow.
In consideration of the above described issues, it is an object of the present invention to provide a low-cost optical network configuration for use primarily in medium-scale IP networks. It is a further object of the present invention to provide optical networks that will                provide the client with the kind of signal capacity demanded by fast-changing IP networks.        provide the flexibility to support communication formats such as SONET and Ethernet        require a lower initial capital investment        provide sufficient expandability for growth, and        provide steady cash flow.        
It is a further object of the present invention to reduce total costs by making effective use of the limited bandwidth resources of optical networks. For optical transmission over distances in excess of 40 km, optical repeaters such as optical fiber amplifiers are required. In recent years, the use of EDFA (Erbium-Doped Fiber Amplifiers, that is made from silica fibers doped with erbium) as optical amplifiers has become commonplace. These optical amplifiers for a wavelength band range of 1530-1560 nm (C-band) have simple components, while it is technically feasible to build EDFAs that will amplify across the entire L-band (1570-1610 nm), the optical amplifiers for this range have complex components. In one possible optical network configuration for that wavelength band range, unless high-precision and expensive components known as wavelength lockers are used, the minimum wavelength separation that can be obtained is around 200 GHz. Therefore, in terms of a power of 2 the number of possible C-band or L-band wavelengths is about 16 channels or wavelengths at most. A way to make effective use of this limited [bandwidth] resource is desired.
It is also an object of the present invention to provide optical networks in which common components are used regardless of the types of signals connectable to the client, the types of signal transmission paths, or the usable bandwidth. Dispersion-shifted optical fibers are used in a transmission path to shift the zero-dispersion wavelength to near 1552 nm, where less expensive C-band optical components can be used. However, when a C-band wavelength division multiplex signal with equal wavelength spacing between channels is transmitted over 40 km in such a path at the normal optical transmission path levels recommended in ITU-T, a phenomenon known as ‘four wave mixing’ (an interference mode between two wavelengths) can occur. Four wave mixing causes overlapping of equally spaced signals, which degrades the optical transmission characteristics of the path. The problem, then, is to devise a way of using low-cost C-band optical components for wavelength-division multiplex transmission in dispersion-shifted optical fiber. One technology already established for eliminating the effects of the four wave mixing phenomenon is ‘unequal spacing.’ Because this technology requires specially designed optical components, it is not conducive to cost reduction. These are the issues that must be improved.